Fabricating current-confining structures in phase change memory switch cells

ABSTRACT

In one or more embodiments, methods of fabricating current-confining stack structures in a phase change memory switch (PCMS) cell are provided. One embodiment shows a method of fabricating a PCMS cell with current in an upper chalcogenide confined in the row and column directions. In one embodiment, methods of fabricating a PCMS cell with sub-lithographic critical dimension memory chalcogenide are shown. In another embodiment, methods of fabricating a PCMS cell with sub-lithographic critical dimension middle electrode heaters are disclosed.

CROSS-REFERENCE TO RELATED PATENT APPLICATION

The present patent application is a divisional patent application ofU.S. patent application Ser. No. 12/646,267 and filed Dec. 23, 2009, thedisclosure of which is incorporated by reference herein.

BACKGROUND

Phase change memory devices use phase change materials for non-volatilestorage in electronic memory applications. Phase change materials may beelectrically switched between a generally amorphous and a generallycrystalline state. One type of memory element utilizes a phase changematerial that may be, in one application, electrically switched betweena structural state of generally amorphous and generally crystallinelocal order or between different detectable states of local order acrossthe entire spectrum between completely amorphous and completelycrystalline states. The phase change materials are non-volatile in that,when set in a crystalline, semi-crystalline, amorphous, orsemi-amorphous state representing a resistance value, that value isretained until changed by another programming event. The phase changematerials remain in the set phase or physical state of the value thephase change material represents.

Power consumption in phase change memory devices can be significant.Reducing the power consumption in phase change memory devices is adesign consideration, especially for use in portable electronic devices.Another consideration during the design of phase change memory devicesincludes dealing with the complexity of phase change materials insemiconductor processing. Further, switch devices in phase change memorydevices are not physically isolated and contribute to cell-to-celldisturb, thus limiting scaling of architecture.

BRIEF DESCRIPTION OF THE DRAWINGS

The claimed subject matter will be understood more fully from thedetailed description given below and from the accompanying drawings ofdisclosed embodiments which, however, should not be taken to limit theclaimed subject matter to the specific embodiment(s) described, but arefor explanation and understanding only.

It will be appreciated that for simplicity and clarity, the figures andthe elements therein are not necessarily drawn to scale. Some featuresin the figures may be exaggerated to show particular details.

FIG. 1 is a schematic diagram of a portion of a memory in accordancewith one embodiment.

FIG. 2 is a cross-sectional view depicting an intermediate structureduring fabrication of a portion of a memory cell of FIG. 1 in accordancewith one embodiment.

FIG. 3 is a cross-sectional view of the structure of FIG. 2 at asubsequent stage of fabrication in accordance with one embodiment.

FIG. 4 is a cross-sectional view of the structure of FIG. 3 at asubsequent stage of fabrication in accordance with one embodiment.

FIG. 5 is a cross-sectional view of the structure of FIG. 4 at asubsequent stage of fabrication in accordance with one embodiment.

FIG. 6 is a cross-sectional view of the structure of FIG. 5 at asubsequent stage of fabrication in accordance with one embodiment.

FIG. 7 is a cross-sectional view of the structure of FIG. 6 through lineA-A′ in accordance with one embodiment.

FIG. 8 is a cross-sectional view of the structure of FIG. 6 through lineA-A′ in accordance with one embodiment.

FIG. 9 is a top view of the structures of FIGS. 7-8 in accordance withvarious embodiments.

FIG. 10 is a cross-sectional view of the structure of FIG. 6 throughline A-A′ in accordance with one embodiment.

FIG. 11 is a top view of the structure of FIG. 10 in accordance with oneembodiment.

FIG. 12 is a cross-sectional view depicting an intermediate structureduring fabrication of a portion of a memory cell of FIG. 1 in accordancewith one embodiment, including a spacer.

FIG. 13 is a cross-sectional view of the structure of FIG. 12 at asubsequent stage of fabrication in accordance with one embodiment.

FIG. 14 is a cross-sectional view of the structure of FIG. 13 at asubsequent stage of fabrication in accordance with one embodiment.

FIG. 15 is a cross-sectional view of the structure of FIG. 14 at asubsequent stage of fabrication in accordance with one embodiment.

FIG. 16 is a cross-sectional view of the structure of FIG. 15 at asubsequent stage of fabrication in accordance with one embodiment.

FIG. 17 is a cross-sectional view of the structure of FIG. 16 at asubsequent stage of fabrication in accordance with one embodiment.

FIG. 18 is a cross-sectional view of the structure of FIG. 17 at asubsequent stage of fabrication in accordance with one embodiment.

FIG. 19 is a cross-sectional view of the structure of FIG. 18 throughline B-B′ in accordance with one embodiment.

FIG. 20 is a cross-sectional view depicting an intermediate structureduring fabrication of a portion of a memory cell of FIG. 1 in accordancewith one embodiment, using trimming.

FIG. 21 is a cross-sectional view of the structure of FIG. 20 at asubsequent stage of fabrication in accordance with one embodiment.

FIG. 22 is a cross-sectional view of the structure of FIG. 21 at asubsequent stage of fabrication in accordance with one embodiment.

FIG. 23 is a cross-sectional view of the structure of FIG. 22 at asubsequent stage of fabrication in accordance with one embodiment.

FIG. 24 is a cross-sectional view of the structure of FIG. 23 throughline C-C′ in accordance with one embodiment.

FIG. 25 is a cross-sectional view depicting an intermediate structureduring fabrication of a portion of a memory cell of FIG. 1 in accordancewith one embodiment, using trimming.

FIG. 26 is a cross-sectional view of the structure of FIG. 25 at asubsequent stage of fabrication in accordance with one embodiment.

FIG. 27 is a cross-sectional view of the structure of FIG. 26 at asubsequent stage of fabrication in accordance with one embodiment.

FIG. 28 is a cross-sectional view depicting an intermediate structureduring fabrication of a portion of a memory cell of FIG. 1 in accordancewith one embodiment, including a spacer.

FIG. 29 is a cross-sectional view of the structure of FIG. 28 at asubsequent stage of fabrication in accordance with one embodiment.

FIG. 30 is a cross-sectional view of the structure of FIG. 29 at asubsequent stage of fabrication in accordance with one embodiment.

FIG. 31 is a cross-sectional view of the structure of FIG. 30 at asubsequent stage of fabrication in accordance with one embodiment.

FIG. 32 is a cross-sectional view of the structure of FIG. 31 at asubsequent stage of fabrication in accordance with one embodiment.

FIG. 33 is a cross-sectional view of the structure of FIG. 32 at asubsequent stage of fabrication in accordance with one embodiment.

FIG. 34 is a cross-sectional view of the structure of FIG. 33 at asubsequent stage of fabrication in accordance with one embodiment.

FIG. 35 is a cross-sectional view of the structure of FIG. 34 throughline D-D′ in accordance with one embodiment.

FIG. 36 is a cross-sectional view depicting an intermediate structureduring fabrication of a portion of a memory cell of FIG. 1 in accordancewith one embodiment, using trimming.

FIG. 37 is a cross-sectional view of the structure of FIG. 36 at asubsequent stage of fabrication in accordance with one embodiment.

FIG. 38 is a cross-sectional view of the structure of FIG. 37 at asubsequent stage of fabrication in accordance with one embodiment.

FIG. 39 is a cross-sectional view depicting an intermediate structureduring fabrication of a portion of a memory cell of FIG. 1 in accordancewith one embodiment, using over-etching.

FIG. 40 is a cross-sectional view of the structure of FIG. 39 at asubsequent stage of fabrication in accordance with one embodiment.

FIG. 41 is a cross-sectional view of the structure of FIG. 40 at asubsequent stage of fabrication in accordance with one embodiment.

FIG. 42 is a block diagram illustrating a portion of a system inaccordance with one embodiment.

DETAILED DESCRIPTION

Referring to FIG. 1, a schematic diagram of a portion of a memory 100including a memory array 102 is shown according to one embodiment. As anexample, memory array 102 may include a 3×3 array of memory cells104-112. In one embodiment, each memory cell 104-112 includes a memoryelement 114 and a select device 116. Although a 3×3 array is illustratedin FIG. 1, the scope of the claimed subject matter is not limited inthis respect. Memory 100 may include a larger array of memory cells.

In one embodiment, memory array 102 may be formed on a portion of asubstrate (not shown). The substrate may include a semiconductorsubstrate such as a silicon substrate. Other substrates such as thosecontaining ceramic material, organic material, or glass material, may besuitable for use. The disclosure below details the formation of aportion of memory array 102 including memory cells. Many, and possiblyall, memory cells of memory array 102, along with other integratedcircuit circuitry may be fabricated simultaneously.

Memory 100 may include column lines 118-120 and row lines 122-124 toselect a particular memory cell of the memory array 102 during a programor read operation. In one embodiment, column lines 118-120 and row lines122-124 may be referred to as address lines since these lines may beused to address memory cells 104-112 during programming or reading. Inone embodiment, column lines 118-120 may be referred to as bit lines androw lines 122-124 may be referred to as word lines.

Memory elements 114 may be connected to row lines 122-124 and may becoupled to column lines 118-120 via select devices 116. When aparticular memory cell is selected, voltage potentials may be applied tothe memory cell's associated column line and row line to apply a voltagepotential across the memory cell. In one example, a voltage is appliedto column line 119 and row line 122 to read memory cell 105.

In one embodiment, memory 100 may be referred to as a phase changememory. Memory elements 114 may comprise a phase change material and maybe referred to as phase change memory elements in one embodiment. Thephase change material may be a material having electrical propertiessuch as, for example, resistance, capacitance, etc. These properties maybe changed through the application of energy such as, for example, heat,light, voltage potential, or electrical current. Examples of a phasechange material may include a chalcogenide and an ovonic material. Thechalcogenide may be a material that includes at least one element fromcolumn VI of the periodic table or may be a material that includes oneor more of the chalcogen elements. For example, chalcogen elements mayinclude tellurium (Te), sulfur (S), or selenium (Se). The ovonicmaterial may be a material that undergoes electronic or structuralchanges and acts as a semiconductor when subjected to the application ofenergy. The ovonic material may be used in memory element 114 or inselect device 116.

As used herein, in general, “chalcogenide” may be used either as a phasechange memory material or a switching material. When chalcogenide is inmemory element 114 and utilized as phase change memory material,chalcogenide may be referred to as “memory chalcogenide”. Whenchalcogenide is used in select device 116 and intended to be a switchingmaterial, chalcogenide may be referred to as “switch chalcogenide”.

Select device 116 may be used to access memory element 114 duringprogramming or reading of memory element 114. Select device 116 mayoperate as a switch that is either “off” or “on” depending on the amountof voltage potential applied across a memory cell. The “off” state maybe a substantially electrically nonconductive state and the “on” statemay be a substantially conductive state. For example, select device 116may have a threshold voltage and if a voltage potential less than thethreshold voltage of select device 116 is applied across select device116, then select device 116 may remain “off” or in a relatively highresistive state so that little or no electrical current passes throughthe memory cell. Alternatively, if a voltage potential greater than thethreshold voltage of select device 116 is applied across select device116, then select device 116 may turn “on”. While select device 116 ison, select device 116 operates in a relatively low resistive state sothat electrical current passes through the memory cell. In other words,select device 116 may be in a substantially electrically nonconductivestate if less than a predetermined voltage potential, such as thethreshold voltage, is applied across select device 116. Select device116 may be in a substantially conductive state if greater than thepredetermined voltage potential is applied across select device 116.Select device 116 may also be referred to as an access device, anisolation device, or a switch.

In one embodiment, select device 116 may comprise a switching materialsuch as, for example, a chalcogenide or an ovonic material, and may bereferred to as an ovonic threshold switch (OTS), or simply an ovonicswitch. The switching material of select device 116 may be a material ina substantially amorphous state positioned between two electrodes, or aninterconnect material and an electrode. The switching material may berepeatedly and reversibly switched between a higher resistance “off”state and a relatively lower resistance “on” state by application of apredetermined electrical current or voltage potential. For example, inthe “off” state, resistance may be greater than about ten mega-ohms, andin contrast, in the “on” state, about zero ohms. In one embodiment,select device 116 may be a two terminal device that may have acurrent-voltage (I-V) characteristic similar to a phase change memoryelement that is in the amorphous state. However, unlike a phase changememory element, the switching material of select device 116 may notchange phase. That is, the switching material of select device 116 maynot be a programmable material, and as a result, select device 116 maynot be a memory device capable of storing information. For example, theswitching material of select device 116 may remain permanently amorphousand the I-V characteristic may remain the same throughout the operatinglife.

During programming of a memory cell, the state of the phase changememory material in the memory cell may be altered. This may be referredto as a “phase-change action”. A voltage potential is applied to a rowinterconnect material and a column interconnect material which generatesa voltage potential between memory element 114 and select device 116. Anelectrical current flows through the phase change memory material inresponse to the applied voltage potential which results in heating ofthe phase change memory material. The phase change memory material maychange state and resistance.

In one convention, the phase change memory material may be in anamorphous state considered the “reset” state. In a crystalline state,the phase change memory material may be considered to be in a “set”state. The resistance of phase change memory material in the amorphousstate may be greater than the resistance of phase change memory materialin the crystalline state. The claimed subject matter is not limited insuch a way, as the opposite of this convention may apply.

Using electrical current, phase change memory material may be heated toa relatively higher temperature to alter phase change memory materialand “reset” the phase change memory material. Heating to a highertemperature allows for programming the phase change memory material to a“0” value. Heating the volume of phase change memory material to arelatively lower crystallization temperature may crystallize phasechange memory material and “set” phase change memory material. Heatingto a lower temperature allows for programming the phase change memorymaterial to a “1” value. Various resistances of phase change memorymaterial may be achieved to store information by varying the amount ofcurrent flow and duration through the volume of phase change memorymaterial.

Referring to FIGS. 2-41, portions of one or more memory cells 104-112are depicted at different stages of fabricating current-confiningstructures in memory cells according to one or more embodiments. Inparticular, FIGS. 2-11 show a fabrication process of a portion of amemory cell including confining current in an upper location memoryelement 114. In multiple embodiments, FIGS. 12-19 show formation of aportion of a memory cell with memory element 114 at sub-criticaldimension (sub-CD) and FIGS. 20-24 and FIGS. 25-27 illustrate other waysof formation of sub-CD memory elements 114. In multiple embodiments,FIGS. 28-35, FIGS. 36-38 and FIGS. 39-41 depict fabrication processes ofa sub-CD heater electrode in contact with memory element 114. In one ormore embodiments, damascene processing may be used in fabricatingcurrent-confining structures. Further, when referring to intermediatestructures, these are generally structures for temporary and/orillustrative purposes and are not intended to refer to permanentstructures.

According to one or more embodiments, current confinement may improvethermal efficiency of a phase-change action, which translates into lowerpower consumption. In one embodiment, upper chalcogenide may be confinedin a row direction as well as a column direction. In one embodiment,minimizing a contact area between an electrode and a phase change memorymaterial may reduce the power required to program a memory cell. Thecontact area may be defined as the surface in which the electrodeinterfaces with the phase change memory material. In one embodiment,minimizing a contact area between a heater electrode and the phasechange memory material may lead to current reduction during programming.This also allows current confinement in structures in accordance withone or more embodiments.

Referring now to FIG. 2, a depiction of an intermediate structure 200during fabrication of a portion of a memory array 102 of FIG. 1 is shownin accordance with one embodiment. Intermediate structure 200 includesrow stack structures 202 that have been patterned and isolation trenchesbetween the row stack structures 202 filled with insulator 204.Conventional etching techniques may be used in patterning the row stackstructures 202. Intermediate structure 200 may be formed above asubstrate.

In a non-limiting example, insulator 204 may include dielectric materialor insulating film that may be formed in between the patterned row stackstructures 202. In one embodiment, dielectric material may includesilicon dioxide (SiO₂), silicon oxide (SiO), silicon nitride (Si₃N₄), orcombinations thereof. In another embodiment, dielectric material mayinclude a material selected with thermal conductivity κ values less than1.0. In one embodiment, insulator 204 may be formed using plasmaenhanced chemical vapor deposition (PECVD), high density plasma chemicalvapor deposition (HDPCVD), spin-on, sol-gel, or other suitableprocesses. In one or more embodiments, insulator 204 may be consideredto be a sacrificial material and/or used as a mask.

According to one embodiment, row stack structure 202 may include layersof a row interconnect material 206, a chalcogenide 208 and a middleelectrode 210. Row stack structure 202 may further include a layer ofchemical barrier material 214 located between row interconnect material206 and chalcogenide 208. In one embodiment, the layers of rowinterconnect material 206, chalcogenide 208 and middle electrode 210 maybe masked and dry etched to pattern the row stack structures 202. In oneexample, an anisotropic etch may be used to pattern the row stackstructures 202. Sacrificial material 212 may be incorporated on top ofrow stack structures 202, which is self-aligned to row by patterningtogether.

In one embodiment, one or more layers of row stack structure 202 may beapplied using a deposition technique. Such deposition techniques includephysical vapor deposition (PVD), chemical vapor deposition (CVD),metal-organic chemical vapor deposition (MOCVD), plasma enhancedchemical vapor deposition (PECVD), high density plasma chemical vapordeposition (HDPCVD), laser assisted chemical vapor deposition, andatomic layer deposition (ALD), but the claimed subject matter is notlimited in this respect. One or more of these deposition techniques maybe selected for appropriate application of various different layersmentioned in this disclosure.

Row interconnect material 206 may include any suitable conductor.Examples of row interconnect material 206 include copper, aluminum,copper alloys, and aluminum alloys. As another example, row interconnectmaterial 206 may be a polycrystalline film with cobalt silicide (CoSi₂)on the top surface. The row interconnect material 206 may correspond torow lines, such as 122-124 of FIG. 1, for addressing memory cells. Avoltage potential between the row interconnect material 206 and a columninterconnect material, to be deposited in a subsequent stage ofmanufacture, may be used to program and read memory cells.

In one embodiment, row interconnect material 206 may have a thicknessranging from about 100 Å to about 20,000 Å. In one embodiment, rowinterconnect material 206 may have a thickness ranging from about 300 Åto about 5,000 Å. In one embodiment, row interconnect material 206 mayhave a thickness of about 2,000 Å.

According to one embodiment, chalcogenide 208 may be a memorychalcogenide or a switch chalcogenide. As mentioned above, memorychalcogenide may be programmable due to the ability to change phasewhereas switch chalcogenide is not programmable due to its inability tochange phase. If chalcogenide 208 comprises memory chalcogenide, then alater formed chalcogenide in the memory cell comprises a switchchalcogenide, and vice versa, as will be further described below.

In one embodiment, there is flexibility to place memory chalcogenide andswitch chalcogenide in either of lower and upper locations within thememory cell. The lower location refers to a position closer to rowinterconnect material 206 and underlying substrate (not shown) of thememory cell, whereas the upper location refers to a position locatedabove the middle electrode 210. In general, as used herein, “lowerchalcogenide” refers to chalcogenide that is located closer to rowinterconnect material and “upper chalcogenide” refers to chalcogenidethat is located closer to column interconnect material.

In one or more embodiments, if upper chalcogenide is fabricated withmemory chalcogenide, the electrical current density may be preservedthroughout the region near the memory chalcogenide. In one or moreembodiments, if the upper chalcogenide is fabricated with switchchalcogenide, when active, the electrical current may be strictlyconfined within the switch chalcogenide.

Commonly, chalcogenide refers to alloys including at least one elementfrom column VI of the Periodic Table of Elements. Examples ofchalcogenide include, but are not limited to, compositions of the classof tellurium-germanium-antimony (Te_(x)Ge_(y)Sb_(z)). In one embodiment,chalcogenide 208 includes Ge₂Sb₂Te₅.

In one embodiment, chalcogenide 208 may be a switch chalcogenidecomprising tellurium and/or selenium. In another embodiment, switchchalcogenide may comprise silicon (Si), tellurium (Te), arsenic (As),and germanium (Ge), or combinations of these elements. In otherembodiments, a composition for switch chalcogenide may include an alloyof silicon (Si), tellurium (Te), arsenic (As), germanium (Ge), andindium (In) or an alloy of Si, Te, As, Ge, and phosphorous (P), but theclaimed subject matter is not limited in this respect.

According to one embodiment, chalcogenide 208 may be deposited to athickness in the range of about 10 Å to about 2,000 Å. In oneembodiment, the thickness of chalcogenide 208 is in the range of about100 Å to about 1,200 Å. In one embodiment, the thickness is on the orderof about 600 Å.

Middle electrode 210 may be comprised of a thin film material having afilm thickness ranging from about 20 Å to about 2,000 Å. In oneembodiment, the thickness of middle electrode 210 may range from about100 Å to about 1,000 Å. In another embodiment, the thickness of middleelectrode 210 may be about 300 Å. Suitable materials for middleelectrode 210 may include titanium (Ti), titanium nitride (TiN),titanium tungsten (TiW), carbon (C), silicon carbide (SiC), titaniumaluminum nitride (TiAlN), titanium silicon nitride (TiSiN),polycrystalline silicon, tantalum nitride (TaN), some combination ofthese films, or other suitable conductors or resistive conductorscompatible with chalcogenide 208.

When referring to “middle electrode”, it should be understood thatmiddle electrode 210 may be an intermediate or an adjacent electrode andnot necessarily an electrode that is in the middle of the stack,electrodes, etc. Further, middle electrode 210 may include one or moreof top or bottom electrodes for memory chalcogenide and switchchalcogenide. The electrodes of the memory chalcogenide and switchchalcogenide may not be required to be identical in composition. In anexample where memory chalcogenide is located in an upper location, anelectrode on the bottom of memory element 114 of FIG. 1 and an electrodeon the top of select device 116 of FIG. 1 may be considered together tocompose middle electrode 210.

As non-limiting examples, the layer of sacrificial material 212 mayinclude carbon and/or spin-on glass. The sacrificial material 212 may bean etch stop layer for chemical mechanical polishing (CMP). In anotherexample, the sacrificial material 212 may include SiN or polycrystallinesilicon films. In one embodiment, the thickness of the sacrificialmaterial 212 may range from about 20 Å to about 4000 Å, and may dependon the desired thickness of chalcogenide that will be subsequentlydeposited in a trench caused by the vacancy of sacrificial material 212.

The layer of chemical barrier 214 located between row interconnectmaterial 206 and chalcogenide 208 may be a thin film for preventing achemical reaction between column interconnect material 206 andchalcogenide 208. Chemical barrier 214 may comprise Ti, TiN, TiW, C,SiC, TiAlN, TiSiN, polycrystalline silicon, TaN, some combination ofthese, or other suitable conductors or resistive conductors. The layerof chemical barrier 214 may be optional and if the chemical barrier 214is absent from the row stack structures 202, chalcogenide 208 may beapplied on row interconnect material 206 and sandwiched between middleelectrode 210 and row interconnect material 206.

Referring to FIG. 3, the structure of FIG. 2, depicted as 220, after thesacrificial material 212 is removed in accordance with one embodiment.In one embodiment, the sacrificial material 212 may be removed using aselective etch without removal of the underlying middle electrode 210 orinsulator 204. In one embodiment, the sacrificial material 212 beingcarbon is removed leaving behind trenches 222 self-aligned to row stackstructures 202.

With reference to FIG. 4, the structure of FIG. 3 after deposition of alayer of a chalcogenide 232 is depicted as structure 230 in accordancewith one embodiment. Chalcogenide 232 may fill in trenches 222completely and overlie insulator 204. In one embodiment, chalcogenide232 may be deposited to a thickness in the range of about 10 Å to about2,000 Å. In one embodiment, the thickness of chalcogenide 208 is in therange of about 100 Å to about 1,200 Å. In one embodiment, the thicknessis on the order of about 600 Å. Chalcogenide 232 may be applied using adeposition technique such as PVD or CVD according to one embodiment.

Referring to FIG. 5 shows the structure of FIG. 4, depicted as structure240 after removing excess chalcogenide 232 in accordance with oneembodiment. A chemical-mechanical polishing (CMP) process may be used toremove undesired chalcogenide. As the surface of structure 240 isplanarized during the CMP process, chalcogenide 232 that is not in thetrenches 222 is removed and chalcogenide 242 is formed on top of andaligned with the row stack structure 202.

Turning to FIG. 6, at 250, once a planar surface has been obtained onthe structure of FIG. 5, a layer of a chemical barrier 252 may beapplied to the planar surface of the structure 240. A layer of columninterconnect material 254 may be deposited on top of the layer ofchemical barrier 252. The layer of chemical barrier 252 may be a thinfilm for preventing a chemical reaction between chalcogenide 242 andcolumn interconnect material 254. Chemical barrier 252 may comprise Ti,TiN, TiW, C, SiC, TiAlN, TiSiN, polycrystalline silicon, TaN, somecombination of these, or other suitable conductors or resistiveconductors. In one embodiment, chemical barrier 252 is optional and thecolumn interconnect material 254 may be directly formed on the planarsurface of the structure 240.

As non-limiting examples, column interconnect material 254 may includesuitable conductors such as copper, copper alloys, aluminum, andaluminum alloys, and polycrystalline film with cobalt silicide (CoSi₂).Column interconnect material 254 may correspond to column lines, such as118-120 of FIG. 1, for addressing memory cells for programming andreading.

In one embodiment, column interconnect material 206 may have a thicknessranging from about 100 Å to about 20,000 Å. In one embodiment, columninterconnect material 206 may have a thickness ranging from about 300 Åto about 5,000 Å. In one embodiment, column interconnect material 206may have a thickness of about 2,000 Å.

After forming column interconnect material 254, column patterning may beperformed to create column stack structures 256, which are more clearlyshown in FIGS. 7-11. During column patterning, chemical barrier 252 andcolumn interconnect material 254 may be etched in a column-wisedirection (y direction), thus forming column stack structures 256. Inone embodiment, as shown at 260, chalcogenide 242 may also be etched andconsidered to be part of column stack structures 256. In one embodiment,middle electrode 210 may also be cut in a column-wise direction. Forexample, anisotropic etch is suitable for patterning in one or moreembodiments.

FIGS. 7, 8 and 10 show cross-sections of the structure of FIG. 6 throughline A-A′ according to various embodiments. FIGS. 9 and 11 show topviews of the various embodiments. It should be noted that although notshown in these figures, the structure of FIG. 6 may be filled with aninsulator in open or unfilled areas, for example, such as between columnstack structures 256 or 260 so as to reduce current leakage. As anon-limiting example, the insulator may be a dielectric material.

FIG. 7 shows a cross-section of the structure of FIG. 6 after columnetching of chalcogenide 242 and middle electrode 210 according to oneembodiment. Chalcogenide 242 may be confined to a cubical region with asmaller volume than prior to column etching. As a result, electricalcurrent density may be preserved throughout the volume. In theembodiment of FIG. 7, chalcogenide 208 is not etched in a column-wisedirection. The column etch process may be extended to etch intochalcogenide 208, as shown in FIG. 8 in an embodiment where part ofchalcogenide 208 is etched. FIG. 9 is a top view corresponding to bothstructures of FIGS. 7-8 in accordance with various embodiments.

By etching chalcogenide 208 in embodiments shown in FIGS. 8 and 10,current that runs through chalcogenide 208 is said to be confined.Instead of spreading out like current normally does in phase changememory switch (PCMS) devices, the current travels mostly in the verticaldirection (z direction) from the row interconnect material 206 to thecolumn interconnect material 254. The column etch process may extenddown to the chemical barrier 214 as shown in FIG. 9 and the chemicalbarrier 214 may act as an etch stop. In one embodiment, current maytravel through a vertical stack 262 of chalcogenide 242 (upperchalcogenide), middle electrode 210, and chalcogenide 208 (lowerchalcogenide), each having been etched in one or more of row and columndirections. FIG. 11 is a top view of the structure of FIG. 10 inaccordance with one embodiment.

In accordance with one or more embodiments, FIGS. 9 and 11 show aportion of memory array 102. From the top views, a cross-point PCMSarrangement of rows and columns of memory array 102 include memory cellslocated at 270 in each of FIGS. 9 and 11. In one or more embodiments,memory cells may be stacked in the vertical direction to increasestorage density.

In one or more embodiments, to increase current density duringprogramming of the memory cell, a reduced size contact area may beformed between a memory chalcogenide and middle electrode for currentconfinement. For a smaller contact area, the lateral size of the memorychalcogenide may be reduced. In one embodiment, the memory chalcogenidemay be formed such that the lateral size is below critical dimension(CD). CD may be defined as the minimum feature size or a target designrule, such as, for example, in photolithography. Fabricating componentsbelow critical dimension may be referred to as “sub-critical dimension”,“sub-lithographic”, “sub-lithographic CD” or simply “sub-CD”.

In one or more embodiments, FIGS. 12-19, FIGS. 20-24 and FIGS. 25-27show processes of forming sub-CD chalcogenide. FIGS. 12-19 and FIGS.25-27 illustrate how sub-CD chalcogenide may be formed at an upperlocation in one or more memory cells, using an additive process and atrimming process, respectively. FIGS. 20-24 illustrate a process offorming sub-CD chalcogenide at a lower location in one or more memorycells, utilizing trimming.

Referring now to FIG. 12, an intermediate structure 300 duringfabrication of a portion of a memory cell of FIG. 1 in accordance withone or more embodiments is shown. In one embodiment, intermediatestructure 300 may be constructed as shown and described in FIG. 2 usingthe same or similar processes and materials. Intermediate structure 300may include row stack structures 301 including layers of rowinterconnect material 302, a chemical barrier 304, a chalcogenide 306,and a middle electrode 308. Sacrificial material 310 may be incorporatedon top of row stack structures 301 and self-aligned to row stackstructure 301 by patterning together. Insulator 312 may be deposited inbetween row stack structures 301. In one embodiment, chalcogenide 306 isswitch chalcogenide.

Referring to FIG. 13, intermediate structure 320 may be constructed asshown and described in FIG. 3 using the same or similar processes andmaterials. Sacrificial material 310 may be selectively removed using ananisotropic etch process leaving trenches 322 self-aligned to row stackstructures 301.

Turning to FIG. 14, intermediate structure 330 is shown after depositinginsulator 332 into the trenches 322 to create spacers inside trenches322. Insulator 332 may be comprised of silicon dioxide, silicon nitride,silicon oxide, or combinations thereof, as non-limiting examples. Thelayer of insulator 332 may be a conformal layer deposited using CVD,HDPCVD or other thin-film deposition techniques. In one embodiment, thethickness of insulator 332 is in the range of about 50 Å to about 1,000Å. In one embodiment, the thickness is on the order of about 300 Å.

Referring to FIG. 15, as shown by intermediate structure 340, adirectional etch is applied to intermediate structure 330 to removeinsulator 332 in the vertical direction. By using an etching agent thatis selective, the etching agent stops at the middle electrode 308. Asdepicted by spacer 344, only the vertical layers of the insulator 332lining the side walls of trenches 322 remain, and new trenches 342 arecreated. According to one embodiment, the trenches 342 are made to benarrow so that the memory chalcogenide that is filled in the trenches342 may be as narrow as possible.

After creating spacer 344 in the trenches 342, FIGS. 16, 17, and 18 mayuse the same or similar processes and materials as shown at FIGS. 4, 5,and 6, respectively. In FIG. 16, memory chalcogenide 352 may fill in thetrenches 342 using CVD or HDPCVD, as an example. In one embodiment, incontrast to the embodiment shown at FIGS. 4, 5, and 6, a contact areabetween memory chalcogenide 352 and middle electrode 308 formed withspacers 344 is smaller and as a result, current can also be confined toa smaller volume. In FIG. 17, undesired chalcogenide may be removed byCMP, for example. Chalcogenide that is left in the trenches is depictedat 362, and may be considered sub-CD. In one embodiment, the thicknessof chalcogenide 362 may be in the range of about 50 Å to about 1,000 Å.In one embodiment, the thickness is on the order of about 300 Å.

Referring to FIG. 18, once a planar surface has been obtained after CMP,a chemical barrier 372 may be deposited on top of the planar surface anda column interconnect material 374 may be deposited on top of thechemical barrier 372 to form structure 370. In one embodiment, chemicalbarrier 372 is optional and the column interconnect material 374 may bedirectly deposited onto the planarized surface. Column patterning may bedone with etching in the column-wise direction (y direction), and thuscreating column stack structures 376. In one embodiment, column stackstructure 376 may include memory chalcogenide 362 and may be patternedas well. Additional column etching may extend to middle electrode 308.

FIG. 19 is a cross-section of FIG. 18 taken through line B-B′ ofstructure 370. The top view of structure 370 is shown by FIG. 9. In oneembodiment, similar to structure 250 of FIG. 6, structure 370 has theoption of further etching the switch chalcogenide 306 either partially,as shown in top view FIG. 9 and cross-section FIG. 8, or completely, asshown in top view FIG. 11.

Turning to FIG. 20, a trimming method for constructing a sub-CD memorychalcogenide is shown with an intermediate structure 400 in the midst ofa row stack structure patterning process. Intermediate structure 400 mayinclude an interconnect material 402 and a chemical barrier 404overlying interconnect material 402, which have not been etched in therow direction (x direction), hence row stack structure is not yetdefined. Chemical barrier 404 may be an optional layer. Intermediatestructure 400 further includes a layer of memory chalcogenide 406 on topof the chemical barrier 404, middle electrode 408 on top of memorychalcogenide 406, and insulator 410 on top of middle electrode 408,which have gone through row patterning. As an etch mask, insulator 410may include a dielectric material such as silicon dioxide or acombination of several insulating films. During patterning, memorychalcogenide 406 is etched and the sidewalls are exposed.

Referring now to FIG. 21, the patterning performed on memorychalcogenide 406 has allowed lateral access to memory chalcogenide 406.Using selective etching, memory chalcogenide 406 may be etchedlaterally, as shown by the arrows. In one embodiment, vacuum conditionsmay be changed in the selective etching process in order to cut memorychalcogenide 406. Chemical barrier 404 is not etched at this stage.

In FIG. 22, the rest of row stack structure 422 including chemicalbarrier 404 and row interconnect material 402 is patterned. The rowstack structure 422 and layers of middle electrode 408 and insulator 410are shown as an intermediate structure 420.

In FIG. 23, according to one embodiment, cross-point PCMS cell processmay occur, as described above, to construct structure 430. In oneembodiment, intermediate structure 420 may be filled in with additionalinsulator 410 using CVD or HDPCVD, for example. The insulator 410 mayreplace the space that was vacated from the lateral etching of thememory chalcogenide 406. In one embodiment, the contact area betweenmemory chalcogenide 406 and middle electrode 408 is maintained andsmaller than CD in width.

Insulator 410 may be removed from the top of intermediate structure 420,for example, using CMP or isotropic etch processes, until the layer ofinsulator 410 above the row stack structure 422 is removed and/or whenmiddle electrode 408 is reached. Once the surface of intermediatestructure 420 is planarized and middle electrode 408 is exposed at thesurface, the process may proceed to deposition of switch chalcogenide432. Additionally, an optional layer of chemical barrier 434 may bedeposited on top of switch chalcogenide 432. Column interconnectmaterial 436 may be formed on top of chemical barrier 434. If chemicalbarrier 434 is not used, column interconnect material 436 may bedeposited on top of switch chalcogenide 432. Column stack structures 438and 440 may be patterned in a column-wise direction. Middle electrode408 may be patterned with column stack structures 438 and 440.

FIG. 24 further shows a cross-section of structure 430, taken throughline C-C′, in accordance with one embodiment. The top view of structure430 is shown by FIG. 9. In one embodiment, similar to structure 250 ofFIG. 6, structure 430 has the option of further etching the memorychalcogenide 406 either partially, as shown in top view FIG. 9 andsimilar to cross-section FIG. 8, or completely (such as down to exposechemical barrier 434) as shown in top view FIG. 11.

Referring to FIGS. 25-27, an alternate method of constructing structure370 of FIG. 18, where memory chalcogenide is sub-CD, is shown usingtrimming. Similar to FIG. 18, an intermediate structure 500 may includea row interconnect material 502, an optional layer of chemical barrier504, switch chalcogenide 506, middle electrode 508, memory chalcogenide510, and insulator 512. During a row patterning process, memorychalcogenide 510 may be etched and the sidewalls are exposed. Bothmemory and switch chalcogenides are present in a row stack structure inone embodiment. With a chemically selective etch, memory chalcogenide510 may be etched laterally, as indicated by the arrows, according toone embodiment. The rest of the row stack structure 520 may be etchedvertically. Similar to FIG. 23, a cross-point PCMS cell processincluding insulator fill and column patterning, as described above, mayoccur following etching of the rest of row stack structure 520 toconstruct structure 370 of FIG. 18.

In one or more embodiments, FIGS. 28-35, FIGS. 36-38 and FIGS. 39-41show processes of forming sub-CD middle electrode heaters. FIGS. 28-35may use an additive process with a damascene approach to forming sub-CDmiddle electrode heaters. FIGS. 36-38 may use a trimming process to formthe heaters. FIGS. 39-41 show another trimming process to form sub-CDheaters. Middle electrode heaters may also be referred to as “heaters”and “heater electrodes”.

In general, current flow between row interconnect material and columninterconnect material results in electrical resistance heat developed bythe heater electrode which heats memory chalcogenide adjacent to theheater electrode. As mentioned above, when memory chalcogenide is heatedto a relatively higher temperature, the memory chalcogenide mayamorphosize (set to “0”). When memory chalcogenide is heated to arelatively lower temperature, the memory chalcogenide may crystallize(set to “1”). In one or more embodiments, when using a sub-CD heater toheat the memory chalcogenide, a higher current density and increaseduniformity of thermal profile within the memory chalcogenide may beachieved.

In forming sub-CD heaters, FIGS. 28 and 29 may use the same or similarprocesses and materials as shown at FIGS. 2 and 3, respectively. FIG. 28shows an intermediate structure 600 including patterned row stackstructures 614 with layers of row interconnect material 602, optionalchemical barrier 604, switch chalcogenide 606, and middle electrode 608.Sacrificial material 610 overlying middle electrode 608 is alsopatterned. Insulator 612 may fill in between the patterned row stackstructures 614 to create intermediate structure 600. At FIG. 29, usingan anisotropic etch process, sacrificial material 610 is removed fromintermediate structure 600 leaving trenches 622 self-aligned to rowstack structures 614.

Turning to FIGS. 30 and 31, in the formation of spacers in trenches 622,FIGS. 30 and 31 may use the same or similar processes and materials asshown at FIGS. 14 and 15, respectively. At FIG. 30, a spacer 632 may bedeposited in trenches 622 and above insulator 612 forming anintermediate structure 630. FIG. 31 shows an intermediate structure 640after directional etching of intermediate structure 630 according to oneembodiment. Anisotropic etching may remove a portion of the spacer 632that is above the middle electrode 608. A selective etching agent mayremove spacer 632 without removing middle electrode 608. Spacers 634remain and create new trenches 642.

Referring to FIG. 32, intermediate structure 650 is shown after forminga heater electrode 652 on top of intermediate structure 640 inaccordance with one embodiment. As non-limiting examples, the heaterelectrode 652 may be TiSiN, Ti, TiN, TiW, C, SiC, TiAlN, polycrystallinesilicon, TaN, other suitable conductors, and combinations thereof. Theheater electrode 652 may be a conformal layer introduced to the top ofintermediate structure 640 using, for example, CVD or HDPCVD. In trench642, a contact area is formed between heater electrode 652 and middleelectrode 608 resulting in a contact area that is sub-CD.

Referring to FIG. 33, in accordance with one embodiment, an intermediatestructure 660 is formed after removing excess heater electrode 652. Inone embodiment, heater electrode 652 may be removed by CMP or isotropicetch. Heater electrode 652 that is left in the trenches 642 is depictedat 662. In one embodiment, the thickness of heater electrode 662 may bein the range of about 50 Å to about 1,000 Å. In one embodiment, thethickness is on the order of about 300 Å.

Referring to FIG. 34, once a planar surface has been obtained, memorychalcogenide 672, chemical barrier 674, and column interconnect material676 may be deposited to the planar surface to form structure 670. In oneembodiment, chemical barrier 674 is optional and the column interconnectmaterial 676 may be directly deposited onto memory chalcogenide 672.Column patterning may be done with anisotropic etching in thecolumn-wise direction, and thus creating column stack structures 678, asshown in FIG. 35. Column stack structures 678 may further include memorychalcogenide 672, which may be etched column-wise. Column etching mayextend down to middle electrode 608.

FIG. 35 is a cross-section of FIG. 34 taken through line D-D′ ofstructure 670. In one embodiment, the top view of structure 670 is shownby FIG. 9. In one embodiment, similar to structure 250 of FIG. 6,structure 670 has the option of further etching the switch chalcogenide606 either partially, as shown in top view FIG. 9 and similar tocross-section FIG. 8, or completely (such as down to expose chemicalbarrier 604) as shown in top view FIG. 11.

Turning to FIG. 36, a depiction of an alternate method, using trimming,to construct structure 670 using the same or similar materials of one ormore embodiments above is shown. In one embodiment, an intermediatestructure 700 may include a row interconnect material 702, chemicalbarrier 704, switch chalcogenide 706, middle electrode 708, heaterelectrode 710, and insulator 712. Chemical barrier 704 may be optionalin one embodiment. During a row patterning process, heater electrode 710may be etched and the sidewalls are exposed. In FIG. 37, a chemicallyselective etch may cut heater electrode 710 laterally, as indicated bythe arrows, according to one embodiment. Referring to FIG. 38, rowpatterning may continue after etching heater electrode 710 so that a rowstack structure 720 may be etched vertically. Similar to FIG. 23, across-point PCMS cell process including insulator fill and columnpatterning, as described above, may complete the trimming process ofconstructing structure 670.

Referring to FIG. 39, a depiction of an alternate method, usingtrimming, to construct an intermediate structure 800 including a sub-CDheater using the same or similar materials of one or more embodimentsabove, is shown. Intermediate structure 800 includes row interconnectmaterial 802, chemical barrier 804, switch chalcogenide 806, middleelectrode 808, heater electrode 810, and insulator 812. Chemical barrier804 may be optional in one embodiment. Row patterning forms row stackstructures 814 in accordance with one embodiment.

In FIG. 40, for trimming purposes, a chemical/physical isotropic etchmay be used to reduce the lateral dimension of the heater electrode 810.In one embodiment, the etching creates a tapered profile on the heaterelectrode 810. In one embodiment, the top surface area of heaterelectrode 810 is smaller than the bottom surface area of heaterelectrode 810. As shown, heater electrode 810 has top surface area atsub-CD while the bottom surface area is at CD. A portion of insulator812 remains on top of heater electrode 810.

Referring to FIG. 41, additional insulator 812 may be filled in betweenthe row stack structures 814. After filling in, insulator 812 may beremoved from the top of intermediate structure 830, for example, usingCMP or isotropic etch processes, until the insulator 812 above the rowstack structures 814 are removed and/or when heater electrodes 822 arereached. Once the surface of intermediate structure 830 is planarizedand heater electrode 822 is exposed at the surface, the process mayproceed to deposition of memory chalcogenide 832. Additionally, anoptional layer of chemical barrier 834 may be deposited on top of memorychalcogenide 832. Column interconnect material 836 may be formed on topof chemical barrier 834. If chemical barrier 834 is not used, columninterconnect material 836 may be deposited on top of memory chalcogenide832. Column stack structure 838 may be patterned in a column-wisedirection. Column stack structure 838 may further include memorychalcogenide 832, which may be etched column-wise as well. Columnetching may extend down to heater electrode 822 and middle electrode808.

An identical cross-section of FIG. 41 taken through line E-E′ ofstructure 830 is shown in FIG. 35. In one embodiment, the top view ofstructure 830 is shown by FIG. 9. In one embodiment, similar tostructure 250 of FIG. 6, structure 830 has the option of further etchingthe switch chalcogenide 806 either partially, as shown in top view FIG.9 and similar to cross-section FIG. 8, or completely (such as down toexpose chemical barrier 804) as shown in top view FIG. 11.

Turning to FIG. 42, a portion of a system 900 in accordance with anembodiment of the present invention is described. System 900 may be usedin wireless devices such as, for example, a personal digital assistant(PDA), a laptop or portable computer with wireless capability, a webtablet, a wireless telephone, a pager, an instant messaging device, adigital music player, a digital camera, or other devices that may beadapted to transmit and/or receive information wirelessly. System 900may be used in any of the following systems: wireless personal areanetworks (WPAN), wireless local area networks (WLAN), and wide areanetworks (WAN) including, for example, 2G cellular networks, 3Gnetworks, and 4G networks, although the scope of the claimed subjectmatter is not limited in this respect.

System 900 may include a controller 902, an input/output (I/O) device904, a memory 906, and a wireless interface 908 coupled to each othervia a bus 910. It should be noted that the scope of the presentinvention is not limited to embodiments having any or all of thesecomponents.

Controller 902 may comprise, for example, one or more microprocessors,digital signal processors, microcontrollers, or the like. Memory 906 maybe used to store messages transmitted to or by system 900. Memory 906may also optionally be used to store instructions that are executed bycontroller 902 during the operation of system 900, and may be used tostore user data. Memory 906 may be provided by one or more differenttypes of memory. For example, memory 906 may comprise any type of randomaccess memory, a volatile memory, a non-volatile memory such as a flashmemory and/or a memory such as memory 100 discussed herein.

I/O device 904 may be used by a user to generate a message. Otherdevices may include a keypad, display, microphone, mouse, camera,sensor, and other electronic devices. System 900 may use wirelessinterface 908 to transmit and receive messages to and from a wirelesscommunication network with radio frequency (RF), microwave, or infraredsignals. Examples of wireless interface 908 may include one or moreantennae or wireless transceivers, although the scope of the presentinvention is not limited in this respect.

It is appreciated that fabrication of current-confining structures ofPCMS cells has been explained with reference to one or more exemplaryembodiments, and that the claimed subject matter is not limited to thespecific details given above. References in the specification made toother embodiments fall within the scope of the claimed subject matter.

In one or more embodiments described above, regarding minimizing contactarea, memory chalcogenide and middle electrode do not physically need tobe in contact, electrical communication between a memory chalcogenideand a middle electrode may be sufficient. In one or more embodiments,electrical communication between a memory chalcogenide and a heaterelectrode may be sufficient.

The figures as shown and described above are not necessarily limited tothe presented layers of materials. For example, a memory cell structuremay include other materials and/or additional layers of electrodes,insulators, chemical barriers, etch stops, chalcogenide, etc. In anotherexample, chemical barriers have been shown in each embodiment, but arenot necessarily required in each embodiment. Further, the figures arenot necessarily limited to the order in which processes occur. In one ormore embodiments, the actual sequence may occur out of order with whatis presented and may still be within the scope of the claimed subjectmatter.

Any reference to “up”, “down”, “row”, “column”, “lateral”, etc. was madeto aid in illustrating one or more embodiments, and is not meant to belimiting in how memory 100 is to be oriented or utilized. In oneexample, references to “row” and “column” may be switched.

Reference in the specification to “an embodiment,” “one embodiment,”“some embodiments,” or “other embodiments” means that a particularfeature, structure, or characteristic described in connection with theembodiments is included in at least some embodiments, but notnecessarily all embodiments, of the claimed subject matter. The variousappearances of “an embodiment,” “one embodiment,” or “some embodiments”are not necessarily all referring to the same embodiments.

If the specification states a component, feature, structure, orcharacteristic “may”, “might”, or “could” be included, that particularcomponent, feature, structure, or characteristic is not required to beincluded. If the specification or claim refers to “a” or “an” element,that does not mean there is only one of the element. If thespecification or claims refer to “an additional” element, that does notpreclude there being more than one of the additional element.

Those skilled in the art having the benefit of this disclosure willappreciate that many other variations from the foregoing description anddrawings may be made within the scope of the claimed subject matter.Indeed, the claimed subject matter is not limited to the detailsdescribed above. Rather, it is the following claims including anyamendments thereto that define such scope and variations. Further, theorder in which the claim elements are presented is not intended to belimiting to the scope of the claimed subject matter.

1. A method of fabricating a phase change memory cell, the methodcomprising: patterning a row stack structure, the row stack structureincluding a row of an interconnect material, a first chalcogenide, and amiddle electrode; forming a sacrificial material; forming an insulatorto isolate the row stack structure from adjacent row stack structures;etching the sacrificial material; forming a second chalcogenide; andpatterning a column stack structure, the column stack structureorthogonal to the row stack structure.
 2. The method of claim 1 whereinsaid etching the sacrificial material comprises leaving behind a trenchself-aligned with the row stack structure and wherein the secondchalcogenide fills the trench.
 3. The method of claim 1 furthercomprising removing excess of the second chalcogenide.
 4. The method ofclaim 1 wherein said patterning a column stack structure comprisesforming one or more layers of a column interconnect material and achemical barrier.
 5. The method of claim 1 further comprising etching ina column-wise direction one or more of the second chalcogenide and themiddle electrode with said patterning a column stack structure.
 6. Themethod of claim 1 further comprising etching in a column-wise directionthe first chalcogenide exposed between the column stack structure and anadjacent column stack structure.
 7. The method of claim 6 wherein saidetching comprises etching a portion of the first chalcogenide exposedbetween the column stack structure and adjacent column stack structure.8. The method of claim 1 wherein the first chalcogenide is a memorychalcogenide and the second chalcogenide is a switch chalcogenide. 9.The method of claim 1 wherein the first chalcogenide is a switchchalcogenide and the second chalcogenide is a memory chalcogenide. 10.The method of claim 1 wherein said forming of a sacrificial materialoccurs on top of the second chalcogenide.
 11. The method of claim 1further comprising lateral etching of the second chalcogenide.
 12. Themethod of claim 1 wherein the sacrificial material is carbon, spin-onglass, or insulator.
 13. The method of claim 1 further comprisingforming a spacer in a trench self-aligned with the row stack structureprior to said forming a second chalcogenide.
 14. The method of claim 13further comprising etching the spacer to expose the middle electrode.15. The method of claim 14 wherein a contact area between the secondchalcogenide and the middle electrode is sub-lithographic criticaldimension.
 16. The method of claim 13 wherein the spacer comprisessilicon nitride, silicon dioxide, silicon oxide, or combinationsthereof.
 17. The method of claim 1 wherein said patterning of row stackstructure comprises lateral etching of the first chalcogenide such thata contact area between the first chalcogenide and the middle electrodeis sub-lithographic critical dimension.
 18. The method of claim 1further comprising column-wise etching of the first chalcogenide and themiddle electrode. 19-22. (canceled)
 23. A method of fabricating a phasechange memory cell, the method comprising: patterning a row stackstructure, the row stack structure comprising a row of an interconnectmaterial, a first chalcogenide, and a middle electrode; forming asacrificial material; forming an insulator to isolate the row stackstructure from adjacent row stack structures; etching the sacrificialmaterial; forming a heater electrode on the middle electrode; andpatterning a column stack structure, wherein the column stack structureis orthogonal to the row stack structure, and wherein the column stackstructure comprises a second chalcogenide.
 24. The method of claim 23wherein the first chalcogenide comprises switch chalcogenide and thesecond chalcogenide comprises memory chalcogenide.
 25. The method ofclaim 24 wherein a contact area between the heater electrode and thememory chalcogenide is sub-lithographic critical dimension.
 26. Themethod of claim 23 further comprising over-etching a portion of thesacrificial material and a portion of the heater electrode to create atapered profile on the heater electrode.
 27. The method of claim 23wherein a contact area between the heater electrode and the middleelectrode is sub-lithographic critical dimension.
 28. The method ofclaim 23 further comprising forming a spacer in a trench left by thesacrificial material after etching.
 29. The method of claim 28 furthercomprising etching a portion of the spacer to expose the middleelectrode.
 30. The method of claim 23 further comprising etching thefirst chalcogenide exposed between the column stack structure and anadjacent column stack structure.